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The role of the trigeminal nerve in olfaction
EXPERIMENTAL
NEUROLOGY
21,
11-19 (1968)
The Role of the Trigeminai HERBERT
Nerve
in Olfaction
-:
STONE, BEATRIZ WILLIAMS, AND ENRIQUE J. A. CARREGAL l Department of Biobehavioral Sciences, Stanford Research Institute, Menlo Park, California 94025 Received
January 10, 1968
The effect of reversible blocking of the trigeminal nerve on olfactory stimulation was studied in rabbits with chronically implanted electrodes. In one series of experiments, blocking the trigeminal nerves at the level of the ganglia increased olfactory bulb excitability, but no cortical desynchronization was evident on odor presentation. Respiration and heart rate remained relatively unchanged during odor presentation. In the trigqminally blocked animal, olfactory bulbinduced sinusoidal wave activity was significantly increased in frequency and amplitude when compared with this activity in the unblocked animal following odor presentation. The present evidence supports the concept of a central regulatory contra! over olfactory afferent inputs; the trigeminal nerve plays an important role in this system. Introduction
It has been suggested that the trigeminal innervation associated with the nonolfactory mucosa of the nares may participate in the olfactory process in a more complex manner than has been previously suspected (2, 3, 12). Early investigations by Parker and Stabler (10) and Katz and Talbert (6) indicated that highly concentrated irritants and pungent organic compounds were capable of stimulating trigeminal nerve endings. Allen (1) found that conditioned reflexes to odorants were abolished after sectioning the olfactory nerves or removing the olfactory bulbs. More recent electrophysiological studies by Beidler (2) and Tucker (12) demonstrated that the trigeminal nerve endings are stimulated also by nonnoxious chemicals and that the concentrations needed to effect this response can be quite small, sometimes less than those required for an observable olfactory response. These observations led us to undertake experiments involving the physiological as well as behavioral response of the rabbit to reasonably high concentrations of odorants after reversibly blocking the trigeminal nerves at 1 This research was supported in part by Grant No. 2002 FR-05522 from the Division of Facilities Resources, National Institutes of Health, to the Life Sciences Research Area of Stanford Research Institute, and in part by Grant No. lRO1 NB 7866-01 from the National Institute of Neurological Diseases and Blindness. Dr. Carregal’s present address is: Department of Physiology, University of Southern California School of Medicine, Los Angeles, California 90033. 11
12
STONE,
WILLIAMS,
AND
CARREGAL
the level of their ganglia ( 11) . This reversible blocking was accomplishes! by means of localized injection of small amounts of Xylocaine (0.0.5-0.1 ml, 2% lidocaine hydrochloride, Astra Pharmaceutical). ConSrmation of the reversible technique was obtained by bilateral electrical lesions of the nerve. Behavioral reactions, respiration, heart rate, cortical EEG, and olfactory bulb (OB) EEG responses were studied in the awake, restrained rabbit in which electrodes were chronically implanted. Results of these experiments suggested that the rabbit’s behavioral response to an odorant may be controlled to a considerable extent by the trigeminal neural input. These data also suggested that induced activity of the OB (produced by means of odorants at equal to or more than 20 times threshold) is mediated in part by the trigeminal nerve. Simultaneous recording from the anterior sensory motor cortex and the OB following odor presentation yielded a typical cortical arousal response and changes in OB-induced sinusoidal wave activity. Blocking of trigeminal conduction immediately increased OB excitability. Odor stimulation failed to produce the cortical desynchronization but seemed to increase OB excitability. Other stimuli, e.g., noise, would elicit an immediate arousal response. These observations are consistent with results from earlier studies (5.7)) which indicate that a central regulatory system exerts a control on afferent inputs from the OB. In our previous study (11) we operationally defined the animal’s response as “aversive” ; i.e., “hyperlocomotion and movement away from the stimulus, changes in respiratory and heart rate, and cortical arousal response.” Subsequent studies have led us to consider dropping this term because of its vagueness and referring to this phenomenon strictly as an “olfactory-trigeminal (OT)” response. In this paper we present data on the OT response to odorants and an evaluation of OB excitability in the rabbit with chronically implanted electrodes. Material
and
Methods
Two series of experiments were carried out. In Exp. I, six young, male, adult New Zealand white rabbits (2.5-3 kg) with chronically implanted electrodes were tested according to methods described previously (11). A bipolar electrode (No. 32 nichrome wire) was positioned in one OR (approximately 2 mm lateral to the midline) ; one sterile 26gauge cannula was placed in the root of each trigeminal nerve at the level of the ganglia ; and two stainless steel screws with attached nichrome wires were placed in the anterior sensory-motor cortex area of the skull for the cortical EEG records. Respiration was recorded with an impedance pneumograph using Ag-AgC1 surface electrodes ; heart rate was recorded with surface electrodes.
OLFACTION
13
A minimum of 3 days elapsed between operations and the experimental session. Each animal was tested once each week over an &week period. Responses to odor stimuli were monitored before, during. and after the reversible trigeminal block ; each animal served as its own control. Each test period lasted 3-4 hours once the animal was accustomed to the experimental environment; i.e., after 1 hour, when respiration was steady and heart rate was 180-210/min. The odorants were presented to the animal by means of an air-dilution system, described previously ( 11). The technique involved placing the animal in a restraining box enclosed in a second, larger unit containing the odor inlet which was positioned in front of the animal. When a stimulus was not being tested, a constant stream of air flowed through. A stimulus was presented for 10 set with a 5min interval between samples to minimize any possible adaptation effects. Experiment II involved six additional rabbits with chronically implanted bipolar electrodes in both OB for recording spontaneous and induced sinusoidal-wave activity. Three of these also had implanted cannulae at the level of the trigeminal ganglia. The remaining three had monopolar, stainless steel, insulated electrodes implanted in the nerve at the ganglia to produce lesions electrically. At the conclusion of the experiments, electrode placement and lesions were confirmed histologically. Because of the exploratory nature of Exp. II, odor presentation was accomplished by means of a less precise technique than for the first experiment ; i.e., purified stimulus in a Teflon bottle, which, when squeezed, would present a puff of stimulus directly at the animal’s nostrils. Dilution of the stimulus was minimal. Recordings were made when the animals were awake and when they were under pentobarbital (35 mg/kg, iv) anesthesia, before and after trigeminal blockade. The OB activity (Exp. 11) was recorded with a Grass Model III polygraph and appropriate EEG preamplifiers. The paper records were analyzed for amplitude and frequency of the characteristic sinusoidal-wave activity in each of the experimental conditions, for each lo-set period prior to, during, and immediately following odor presentation, These data were subjected to statistical analysis by means of the t test (4) ; p < Cl.05 was considered significant. The odor stimuli included propionic acid, n-propyl alcohol, and toluene. All odorants were redistilled before use and were in excess of 99% purity according to gas chromatographic analysis. For some tests, cigarette smoke was also used. Results
Experiment I results were study : Upon odor presentation
similar to those reported in our previous there were changes in respiratory and heart
11
STONE,
WILLIAMS,
AND
CARREGAL
response, and changes in OB activity. rates, a typical cortical “arousal” Following trigeminal blockade (by Xylocaine or lesion), changes were noted in the cortical response and OB activity before presentation of odor stimuli. The data for respiration and heart rate changes following odor presentation are summarized in Fig. 1. nlso included are data from earlier experiments (11). The changes in heart rate (pre-Xylocaine j were positive for some odorants (i.e., an increase over control), but for others there was a decrease in the rate. For two odorants, toluene and propionic acid, the lower stimulus concentration decreased heart rate whereas the higher concentration tended to increase heart rate. Since only one of the four concentrations (of the two odorants) produced a statistically significant difference (p < 0.05) f rom the control condition, none of these differences may be important. Following trigeminal blockade, respiration and heart rate exhibited little change from baseline responses to odorants. This was consistent with the observed passive behavior of the animals during stimulus presentation. The cortical arousal response, as well as the OB excitation, as noted above, showed an increase in activity. Nonolfactory stimuli, e.g., noise, would elicit an immediate arousal response. In some BEFORE
AFTER
XYLOCAINE
XYLOCAINE
60
*STATISTICALLY
SltNlFlCANT
(p
c. 0.05)
FIG. 1. Bar graph showing percentage changes from prestimulation values for heart rate (upper) and respiratory rate (lower) following odor presentation before and after administration of Xylocaine (0.05-0.1 ml of a 2% lidocaine hydrochloride solution injected into each trigeminal ganglion). Each entry is the mean of 20-25 responses from each of the six test animals.
15
OLFACTION
animals the increased OB spontaneous activity tended to mask the usual cortical response; however, in a sufficient number of experiments the lack of cortical desynchronization was readily apparent. Detailed analysis of OB activity was not possiblein these experiments. The characteristic OB spontaneous activity during odor stimulation has been described in detail for the rabbit in previous investigations by Ottoson (9)) Hernandez-Peon et al. (5)) and Moulton (8). The results of Exp. II (Figs. 2 and 3) confirm that this activity in the awake animal is characterized by sinusoidal waves, which occur in bursts, with an average
H IOOpJ 0.1-i
ec
FIG. 2. Olfactory bulbar response to stimulation by propionic acid (purified stimulus, 100% saturation) under the different experimental conditions: A, animal awake before Xylocaine administration ; B, awake, after Xylocaine administration ; C, anesthetized (pentobarbital, 35 mg/kg, iv), before Xylocaine; D, anesthetized, after Xylocaine; E, awake, before lesions (of the trigeminal ganglia) ; F, awake, 3 days after lesions; G, anesthetized, before lesions ; H, asleep, 3 days after lesions. The upper records are part of the total record from one of the three Xylocaineblocked animals. These results are typical of the responses obtained from the Xylocaine-blocked animals. The lower records are from one of the animals with lesions.
16
STONE,
WILLIAMS,
AND
CARREGAL
Gl H 100,uv -I 0.1 set FIG. 3. Olfactory bulbar response to puffs of cigarette smoke under the different experimental conditions : Same as Fig. 2.
fundamental wave frequency of about 50 cycle/set and an average wave amplitude of about 80 pv per burst. During pentobarbital anesthesia, the wave frequency decreased to about 20 cycle/set, but the amplitude remained about the sameas in the awake animal. Both chronic and reversible blockage of the trigeminal nerve produced similar results in OB activityan increase in frequency and amplitude of the sinusoidal wave induced by odorants when compared with values obtained before the block. In the anesthetized animal, during odor stimulation, the increase in frequency was greater than that obtained in the awake animal, but the amplitude increase was smaller. In both conditions, awake and anesthetized, the changes in frequency and amplitude were significantly different (p < 0.05). These data are summarized in Table 1. Discussion
The results of Exp. I were consistent with our previous findings that the response of the rabbit to odorants is an “olfactory-trigeminal” re-
17
OLFACTION
TABLE FREQUENCY
AND
PRESENTATION
AMPLITUDE OF THE
CHANGES
1
OF THE
OB
SINUSOIDAL
ODORANTS, VVPROPYL ALCOHOL, CIGARETTE SMOKE
WAVES
PROPIONIC
FOLLOWING
ACID,
AND
Awake Control Frequency (cycle/set) Amplitude
Xylocaine
48.6 + 2.11= 6.0 *
60.0 * 3.2s
( +21.2)c
10.8 + 1.2b
(+SO.O)
0.65
(/Jv) Lesion Frequency (cycle/set) Amplitude
45.3 t 2.81
54.8 + 2.707’ ( +20.9)
5.1 f 0.52
8.9 + 1.02s
( +74.5)
(av> Asleep Control Frequency (cycle/set) Amplitude b>
17.5 + 1.21
Frequency (cycle/set) Amplitude (av>
Xylocaine 24.5 t
1.43b
(+40.0)
5.8 + 0.70
8.3 + 0.92b
($43.1)
20.6 -+ 1.42
29.4 k 1.69’
Lesion
4.2 * 0.82
a Each entry is the mean of 20 to 30 responses from b Significance at p < 0.05, by f test (4). C Value in parentheses is the percentage change.
8.4 *
1.03b
( $42.7) (+35.5)
each animal.
sponse. That these odorants did not produce very dramatic changes in heart rate was unexpected. We hesitate to speculate on the meaning of this, in view of the limited number of odorants and test concentrations studied. Of most interest to us were the rather significant changes between pre- and postadministration of the Xylocaine. These findings support the hypothesis that the response to an olfactory stimulus is mediated, in part, by the trigeminal system. Studies by Kerr and Hagbarth (7), Moulton (S), and others have demonstrated that there are numerous complex centrifugal as well as other sensory efferent fiber inputs into the OB. Conceivably this centrifugal pathway could serve as an elaborate monitoring system capable of altering bulbar activity either through inhibition or facilitation of the electrical activity, thus enabling information to be processed more rapidly (13). The present findings lend support to the proposition that the electrical
18
STONE,
WILLIAMS,
AND
CARREGAL
activity of the rabbit OB is altered following trigeminal blockage (Exp. II). These results are consistent with those reported by Kerr and Hagbarth (7) when they sectioned the anterior commissure; i.e., a synchronization of bulbar afferent activity with a much increased amplitude (see their Fig. 8). These investigators also noted that stimulation of the reticular formation inhibited the electrical activity of the OB. From these observations and our own experiments we assume that in the awake animal breathing unfiltered room air, the trigeminal nerves are involved in the inhibitory influences that affect bulbar activity even in the absence of high concentrations of odorants. We are aware that some of the increased excitability in OB activity following trigeminal block could be due to increased accessibility of the olfactory mucosa to odors normally present in room air, since the autonomic reflexes that control accessibility via trigeminal activation are no longer present. However, the fact that no cortical desynchronization was evident during odor stimulation (at least, insofar as we were able to observe in this experimental arrangement) suggests that this increased excitability could be due, in part, to the release of inhibitory influences from the reticular formation. HernSndez-Pe6n et al. (5) have shown that electrical stimulation of the reticular formation produces either inhibitory or facilitory interactions at all the synaptic stations of the specific afferent pathways, including olfactory. Experiments are now in progress in our laboratory to further elucidate this rather complex but seemingly important aspect of olfaction-the interaction of the olfactory and the trigeminal systems. References 1. 2. 3. 4. 5.
6.
7. 8.
ALLEN, W. F. 1937. Olfactory and trigeminal conditioned reflexes in dogs. A+rl. J. Physiol. 118 : 532-540. BEIDLER, L. M. 1965. Comparison of gustatory receptors, olfactory receptors and free nerve endings. Cold Spring Harbor Synzp. Quant. Biol. 30: 191-200. DAWSON, W. 1962. Chemical stimulation of the peripheral trigeminal nerve. Nature 1% : 341-345. GOULDEN, C. H. 1952. “Methods of Statistical Analysis” (2nd ed.), Wiley, New York. HERNANDEZ-PEON, R., A. LAVIN, C. ALCOCEK-CUAR~N, and J. P. MARCELIN. 1960. Electrical activity of the olfactory bulb during wakefulness and sleep. Electroenceplaalog. Clin. Neurophysiol. 12 : 41-58. KATZ, S. H., and E. J. TALBERT. 1930. Intensities of odors and irritating effects of warning agents for inflammable and poisonous gases. US Bureau Mines, Tech. Papers, 480, PP. 37. KQRR, D. I. B., and I;. E. HAGBARTH. 1955. An investigation of olfactory centrifugal fiber system. J. Neurophysiol. 18 : 362-374. MOULTON, D. G. 1963. Electrical activity in the olfactory system of rabbits with indwelling electrodes, pp. 71-84. 112: “Olfaction and Taste.” Y. Zotterman [ed.]. MacMillan, New York.
OLFACTION
19
9. OTTOSON, D. 1959. Studies on slow potentials in the rabbit’s olfactory bulb and nasal mucosa. Acta Physiol. Stand. 47 : 136-148. 10. PARKER, G. H., and E. M. STABLER. 1913. On certain distinctions between taste and smell. Am. J. Physiol. 32 : 230-240. 11. STONE, H., E. J. CARREGAL, and B. WILLIAMS. 1966. The olfactory trigeminal response to odorants. Life Sci. 5 : 2195-2201. 12. TUCKER, D. 1963. Physical variables in the olfactory stimulation process. J. Gem. Physiol.
13. WENZET., 381 4.~4.
46 : 453-489.
R. M.. and M. H. 9 TRCK. 1966. Olfaction.
A+I~L I&W. P!1ysicZ. 28 :
NEUROLOGY
21,
11-19 (1968)
The Role of the Trigeminai HERBERT
Nerve
in Olfaction
-:
STONE, BEATRIZ WILLIAMS, AND ENRIQUE J. A. CARREGAL l Department of Biobehavioral Sciences, Stanford Research Institute, Menlo Park, California 94025 Received
January 10, 1968
The effect of reversible blocking of the trigeminal nerve on olfactory stimulation was studied in rabbits with chronically implanted electrodes. In one series of experiments, blocking the trigeminal nerves at the level of the ganglia increased olfactory bulb excitability, but no cortical desynchronization was evident on odor presentation. Respiration and heart rate remained relatively unchanged during odor presentation. In the trigqminally blocked animal, olfactory bulbinduced sinusoidal wave activity was significantly increased in frequency and amplitude when compared with this activity in the unblocked animal following odor presentation. The present evidence supports the concept of a central regulatory contra! over olfactory afferent inputs; the trigeminal nerve plays an important role in this system. Introduction
It has been suggested that the trigeminal innervation associated with the nonolfactory mucosa of the nares may participate in the olfactory process in a more complex manner than has been previously suspected (2, 3, 12). Early investigations by Parker and Stabler (10) and Katz and Talbert (6) indicated that highly concentrated irritants and pungent organic compounds were capable of stimulating trigeminal nerve endings. Allen (1) found that conditioned reflexes to odorants were abolished after sectioning the olfactory nerves or removing the olfactory bulbs. More recent electrophysiological studies by Beidler (2) and Tucker (12) demonstrated that the trigeminal nerve endings are stimulated also by nonnoxious chemicals and that the concentrations needed to effect this response can be quite small, sometimes less than those required for an observable olfactory response. These observations led us to undertake experiments involving the physiological as well as behavioral response of the rabbit to reasonably high concentrations of odorants after reversibly blocking the trigeminal nerves at 1 This research was supported in part by Grant No. 2002 FR-05522 from the Division of Facilities Resources, National Institutes of Health, to the Life Sciences Research Area of Stanford Research Institute, and in part by Grant No. lRO1 NB 7866-01 from the National Institute of Neurological Diseases and Blindness. Dr. Carregal’s present address is: Department of Physiology, University of Southern California School of Medicine, Los Angeles, California 90033. 11
12
STONE,
WILLIAMS,
AND
CARREGAL
the level of their ganglia ( 11) . This reversible blocking was accomplishes! by means of localized injection of small amounts of Xylocaine (0.0.5-0.1 ml, 2% lidocaine hydrochloride, Astra Pharmaceutical). ConSrmation of the reversible technique was obtained by bilateral electrical lesions of the nerve. Behavioral reactions, respiration, heart rate, cortical EEG, and olfactory bulb (OB) EEG responses were studied in the awake, restrained rabbit in which electrodes were chronically implanted. Results of these experiments suggested that the rabbit’s behavioral response to an odorant may be controlled to a considerable extent by the trigeminal neural input. These data also suggested that induced activity of the OB (produced by means of odorants at equal to or more than 20 times threshold) is mediated in part by the trigeminal nerve. Simultaneous recording from the anterior sensory motor cortex and the OB following odor presentation yielded a typical cortical arousal response and changes in OB-induced sinusoidal wave activity. Blocking of trigeminal conduction immediately increased OB excitability. Odor stimulation failed to produce the cortical desynchronization but seemed to increase OB excitability. Other stimuli, e.g., noise, would elicit an immediate arousal response. These observations are consistent with results from earlier studies (5.7)) which indicate that a central regulatory system exerts a control on afferent inputs from the OB. In our previous study (11) we operationally defined the animal’s response as “aversive” ; i.e., “hyperlocomotion and movement away from the stimulus, changes in respiratory and heart rate, and cortical arousal response.” Subsequent studies have led us to consider dropping this term because of its vagueness and referring to this phenomenon strictly as an “olfactory-trigeminal (OT)” response. In this paper we present data on the OT response to odorants and an evaluation of OB excitability in the rabbit with chronically implanted electrodes. Material
and
Methods
Two series of experiments were carried out. In Exp. I, six young, male, adult New Zealand white rabbits (2.5-3 kg) with chronically implanted electrodes were tested according to methods described previously (11). A bipolar electrode (No. 32 nichrome wire) was positioned in one OR (approximately 2 mm lateral to the midline) ; one sterile 26gauge cannula was placed in the root of each trigeminal nerve at the level of the ganglia ; and two stainless steel screws with attached nichrome wires were placed in the anterior sensory-motor cortex area of the skull for the cortical EEG records. Respiration was recorded with an impedance pneumograph using Ag-AgC1 surface electrodes ; heart rate was recorded with surface electrodes.
OLFACTION
13
A minimum of 3 days elapsed between operations and the experimental session. Each animal was tested once each week over an &week period. Responses to odor stimuli were monitored before, during. and after the reversible trigeminal block ; each animal served as its own control. Each test period lasted 3-4 hours once the animal was accustomed to the experimental environment; i.e., after 1 hour, when respiration was steady and heart rate was 180-210/min. The odorants were presented to the animal by means of an air-dilution system, described previously ( 11). The technique involved placing the animal in a restraining box enclosed in a second, larger unit containing the odor inlet which was positioned in front of the animal. When a stimulus was not being tested, a constant stream of air flowed through. A stimulus was presented for 10 set with a 5min interval between samples to minimize any possible adaptation effects. Experiment II involved six additional rabbits with chronically implanted bipolar electrodes in both OB for recording spontaneous and induced sinusoidal-wave activity. Three of these also had implanted cannulae at the level of the trigeminal ganglia. The remaining three had monopolar, stainless steel, insulated electrodes implanted in the nerve at the ganglia to produce lesions electrically. At the conclusion of the experiments, electrode placement and lesions were confirmed histologically. Because of the exploratory nature of Exp. II, odor presentation was accomplished by means of a less precise technique than for the first experiment ; i.e., purified stimulus in a Teflon bottle, which, when squeezed, would present a puff of stimulus directly at the animal’s nostrils. Dilution of the stimulus was minimal. Recordings were made when the animals were awake and when they were under pentobarbital (35 mg/kg, iv) anesthesia, before and after trigeminal blockade. The OB activity (Exp. 11) was recorded with a Grass Model III polygraph and appropriate EEG preamplifiers. The paper records were analyzed for amplitude and frequency of the characteristic sinusoidal-wave activity in each of the experimental conditions, for each lo-set period prior to, during, and immediately following odor presentation, These data were subjected to statistical analysis by means of the t test (4) ; p < Cl.05 was considered significant. The odor stimuli included propionic acid, n-propyl alcohol, and toluene. All odorants were redistilled before use and were in excess of 99% purity according to gas chromatographic analysis. For some tests, cigarette smoke was also used. Results
Experiment I results were study : Upon odor presentation
similar to those reported in our previous there were changes in respiratory and heart
11
STONE,
WILLIAMS,
AND
CARREGAL
response, and changes in OB activity. rates, a typical cortical “arousal” Following trigeminal blockade (by Xylocaine or lesion), changes were noted in the cortical response and OB activity before presentation of odor stimuli. The data for respiration and heart rate changes following odor presentation are summarized in Fig. 1. nlso included are data from earlier experiments (11). The changes in heart rate (pre-Xylocaine j were positive for some odorants (i.e., an increase over control), but for others there was a decrease in the rate. For two odorants, toluene and propionic acid, the lower stimulus concentration decreased heart rate whereas the higher concentration tended to increase heart rate. Since only one of the four concentrations (of the two odorants) produced a statistically significant difference (p < 0.05) f rom the control condition, none of these differences may be important. Following trigeminal blockade, respiration and heart rate exhibited little change from baseline responses to odorants. This was consistent with the observed passive behavior of the animals during stimulus presentation. The cortical arousal response, as well as the OB excitation, as noted above, showed an increase in activity. Nonolfactory stimuli, e.g., noise, would elicit an immediate arousal response. In some BEFORE
AFTER
XYLOCAINE
XYLOCAINE
60
*STATISTICALLY
SltNlFlCANT
(p
c. 0.05)
FIG. 1. Bar graph showing percentage changes from prestimulation values for heart rate (upper) and respiratory rate (lower) following odor presentation before and after administration of Xylocaine (0.05-0.1 ml of a 2% lidocaine hydrochloride solution injected into each trigeminal ganglion). Each entry is the mean of 20-25 responses from each of the six test animals.
15
OLFACTION
animals the increased OB spontaneous activity tended to mask the usual cortical response; however, in a sufficient number of experiments the lack of cortical desynchronization was readily apparent. Detailed analysis of OB activity was not possiblein these experiments. The characteristic OB spontaneous activity during odor stimulation has been described in detail for the rabbit in previous investigations by Ottoson (9)) Hernandez-Peon et al. (5)) and Moulton (8). The results of Exp. II (Figs. 2 and 3) confirm that this activity in the awake animal is characterized by sinusoidal waves, which occur in bursts, with an average
H IOOpJ 0.1-i
ec
FIG. 2. Olfactory bulbar response to stimulation by propionic acid (purified stimulus, 100% saturation) under the different experimental conditions: A, animal awake before Xylocaine administration ; B, awake, after Xylocaine administration ; C, anesthetized (pentobarbital, 35 mg/kg, iv), before Xylocaine; D, anesthetized, after Xylocaine; E, awake, before lesions (of the trigeminal ganglia) ; F, awake, 3 days after lesions; G, anesthetized, before lesions ; H, asleep, 3 days after lesions. The upper records are part of the total record from one of the three Xylocaineblocked animals. These results are typical of the responses obtained from the Xylocaine-blocked animals. The lower records are from one of the animals with lesions.
16
STONE,
WILLIAMS,
AND
CARREGAL
Gl H 100,uv -I 0.1 set FIG. 3. Olfactory bulbar response to puffs of cigarette smoke under the different experimental conditions : Same as Fig. 2.
fundamental wave frequency of about 50 cycle/set and an average wave amplitude of about 80 pv per burst. During pentobarbital anesthesia, the wave frequency decreased to about 20 cycle/set, but the amplitude remained about the sameas in the awake animal. Both chronic and reversible blockage of the trigeminal nerve produced similar results in OB activityan increase in frequency and amplitude of the sinusoidal wave induced by odorants when compared with values obtained before the block. In the anesthetized animal, during odor stimulation, the increase in frequency was greater than that obtained in the awake animal, but the amplitude increase was smaller. In both conditions, awake and anesthetized, the changes in frequency and amplitude were significantly different (p < 0.05). These data are summarized in Table 1. Discussion
The results of Exp. I were consistent with our previous findings that the response of the rabbit to odorants is an “olfactory-trigeminal” re-
17
OLFACTION
TABLE FREQUENCY
AND
PRESENTATION
AMPLITUDE OF THE
CHANGES
1
OF THE
OB
SINUSOIDAL
ODORANTS, VVPROPYL ALCOHOL, CIGARETTE SMOKE
WAVES
PROPIONIC
FOLLOWING
ACID,
AND
Awake Control Frequency (cycle/set) Amplitude
Xylocaine
48.6 + 2.11= 6.0 *
60.0 * 3.2s
( +21.2)c
10.8 + 1.2b
(+SO.O)
0.65
(/Jv) Lesion Frequency (cycle/set) Amplitude
45.3 t 2.81
54.8 + 2.707’ ( +20.9)
5.1 f 0.52
8.9 + 1.02s
( +74.5)
(av> Asleep Control Frequency (cycle/set) Amplitude b>
17.5 + 1.21
Frequency (cycle/set) Amplitude (av>
Xylocaine 24.5 t
1.43b
(+40.0)
5.8 + 0.70
8.3 + 0.92b
($43.1)
20.6 -+ 1.42
29.4 k 1.69’
Lesion
4.2 * 0.82
a Each entry is the mean of 20 to 30 responses from b Significance at p < 0.05, by f test (4). C Value in parentheses is the percentage change.
8.4 *
1.03b
( $42.7) (+35.5)
each animal.
sponse. That these odorants did not produce very dramatic changes in heart rate was unexpected. We hesitate to speculate on the meaning of this, in view of the limited number of odorants and test concentrations studied. Of most interest to us were the rather significant changes between pre- and postadministration of the Xylocaine. These findings support the hypothesis that the response to an olfactory stimulus is mediated, in part, by the trigeminal system. Studies by Kerr and Hagbarth (7), Moulton (S), and others have demonstrated that there are numerous complex centrifugal as well as other sensory efferent fiber inputs into the OB. Conceivably this centrifugal pathway could serve as an elaborate monitoring system capable of altering bulbar activity either through inhibition or facilitation of the electrical activity, thus enabling information to be processed more rapidly (13). The present findings lend support to the proposition that the electrical
18
STONE,
WILLIAMS,
AND
CARREGAL
activity of the rabbit OB is altered following trigeminal blockage (Exp. II). These results are consistent with those reported by Kerr and Hagbarth (7) when they sectioned the anterior commissure; i.e., a synchronization of bulbar afferent activity with a much increased amplitude (see their Fig. 8). These investigators also noted that stimulation of the reticular formation inhibited the electrical activity of the OB. From these observations and our own experiments we assume that in the awake animal breathing unfiltered room air, the trigeminal nerves are involved in the inhibitory influences that affect bulbar activity even in the absence of high concentrations of odorants. We are aware that some of the increased excitability in OB activity following trigeminal block could be due to increased accessibility of the olfactory mucosa to odors normally present in room air, since the autonomic reflexes that control accessibility via trigeminal activation are no longer present. However, the fact that no cortical desynchronization was evident during odor stimulation (at least, insofar as we were able to observe in this experimental arrangement) suggests that this increased excitability could be due, in part, to the release of inhibitory influences from the reticular formation. HernSndez-Pe6n et al. (5) have shown that electrical stimulation of the reticular formation produces either inhibitory or facilitory interactions at all the synaptic stations of the specific afferent pathways, including olfactory. Experiments are now in progress in our laboratory to further elucidate this rather complex but seemingly important aspect of olfaction-the interaction of the olfactory and the trigeminal systems. References 1. 2. 3. 4. 5.
6.
7. 8.
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